1. Field
The disclosed concept pertains generally to arc fault detection and, more particularly, to direct current arc fault circuit interrupters. The disclosed concept also pertains to direct current arc fault detectors, noise blanking circuits for direct current arc fault circuit interrupters, and methods of detecting arc faults.
2. Background Information
It is believed that there is no known mechanism in photovoltaic (PV) (e.g., photovoltaic; solar electric) systems to detect arcing faults and stop strings or string arrays from generating energy under an in-circuit (series) fault or a short circuit fault (e.g., without limitation, a parallel arc), which can result in a fire. For example, fuses at the load end of a string do not prevent this fault. For example, arcs consume energy that does not transfer to an inverter or load.
Known practice places a protective device (i.e., a fuse) at the load end of a string, in one feed conductor (e.g., wire; typically the positive wire) to open when back feed currents exceed 150% of the fuse rating during back feed conditions and back feed shorts. Depending on the manufacturer, either the positive or negative feed wire will contain a protective device (i.e., a fuse). Depending on the local building codes, the system may have a grounded conductor or may be un-grounded. Some known combiner boxes include fuses on both conductors for ungrounded systems at the feed end but not at the remote end. It is believed that protective devices are not used at the PV generating modules, at the remote end of a string, or in the return conductor. It is believed that all of the connecting feed conductors between the PV generating modules and the return conductors are un-protected from arcing events or short circuits of many kinds
FIG. 1 shows several parallel strings 2,4,6 of series-connected direct current (DC) electrical generating modules 8 (e.g., PV generating modules) with a protective device 10 located in the positive conductor 12 of each string. This protective device 10 is a fuse and only protects against a reverse over current when the corresponding string 2,4,6 shorts and is back fed by the other PV strings which are bussed together at the main DC bus 14 in the combiner box 16.
It is known to employ fuses for over current protection and diodes to block reverse current. It is believed that known strings and arrays of DC electrical generating modules do not provide series or parallel arc fault protection.
It is believed that fuses are essentially useless in PV power systems since such fuses are sized at 125% and typically must open at 150% of full load current, while the maximum short circuit current for PV arrays does not exceed 100% of full load current. In PV power systems, fuses protect conductors, such as wires, from over currents. Over currents can only result from a back feed condition. Forward feed currents for silicon PV modules are typically limited to about 7.5 A by the structure of the PV modules, which cannot produce more than about 7.5 A even when short circuited. Therefore, string fuses are useless for providing protection from a forward feed fault, or bus faults that occur above the string fuses.
It is believed that fuses also cannot provide protection from an arcing fault. A fuse is typically used on each string, which is a series connected number of PV modules to achieve, for example, a 600 VDC potential. The string is terminated in a central junction box commonly referred to as a combiner box.
Module DC/DC converters can turn off power to exposed connectors, but are believed to lack a detection mechanism for faults. Otherwise, there is no protection or control at the PV module and, therefore, no way to turn off the potential generated by the PV modules other than to block the light and prevent illumination of the silicon diodes of the PV module by covering the PV modules, an impractical strategy in the field.
DC/DC converters can help prevent backfed faults as they maintain voltage consistency between the voltage provided by the string and the voltage required by the DC bus to reach the maximum power point (MPP). However, there is no protection for series arcing faults or many short circuits between the positive and negative feed wires or ground.
There is room for improvement in direct current arc fault circuit interrupters.
There is also room for improvement in direct current arc fault detectors.
There is further room for improvement in methods of detecting arc faults.